1. Field of the Invention
The invention relates to a method and apparatus for manufacturing metal alloys, more particularly to a method and apparatus for manufacturing a light metal alloy by the process of injection molding the metal alloy when it is in a thixotropic (semi-solid) state.
2. Description of the Related Art
One conventional method used to produce molds of metal alloys is the die cast method. The die cast method is disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298, both of which are incorporated by reference herein. The die cast method uses liquid metal alloys during casting and as a consequence, metal alloys produced from this method have low densities. Metal alloys having low densities are not desirable because of their lower mechanical strength, higher porosity, and larger micro shrinkage. It is thus difficult to accurately dimension molded metal alloys, and once dimensioned, to maintain their shapes. Moreover, metal alloys produced from die casting have difficulty in reducing the resilient stresses developed therein.
The thixotropic method improves upon the die casting method by injection molding a metal alloy from its thixotropic (semi-solid) state rather than die casting it from its liquid state. The result is a metal alloy which has a higher density than one produced from the die casting method.
A method and apparatus for manufacturing a metal alloy from its thixotropic state is disclosed in U.S. Pat. No. 5,040,589, which is incorporated by reference herein. A method of converting a metal alloy into a thixotropic state by controlled heating is disclosed in U.S. Pat. Nos. 4,694,881 and 4,694,882, both of which are incorporated by reference herein.
The system disclosed in U.S. Pat. No. 5,040,589 is an in-line system, in which the conversion of the metal alloy into a thixotropic state and the pressurizing of the same for the purposes of injection molding is carried out within a single cylindrical housing. With such a system, it is difficult to control the molding conditions, i.e., temperature, pressure, time, etc., and as a result, metal alloys of inconsistent characteristics are produced.
Moreover, the system of U.S. Pat. No. 5,040,589 requires that the metal alloy supplied to the feeder be in pellet form. As a consequence, if a mold of undesired characteristics are produced by its system, recycling of the defective molds is not possible unless the defective molds are recast in pellet form.
An improved system for manufacturing light alloy metals, which is capable of accurately producing molded metal alloys of specified dimensions within a narrow density tolerance, is desired. Further, a production process for light alloy metals which can consistently produce molded metal alloys of desired characteristics, and which can easily accommodate recycling of defective molds would represent a substantial advance in this art.